The present invention relates to recording on a medium, and more specifically, to laser control during recording on an optical medium.
Before user data is recorded, a test write is performed on a small segment of the media. For optical disks, this segment is along the inner radius of the disk. The test write writes at various power levels and write pulse shapes. By reading back the test written pattern, the optimum power level and pulse shape are set, for recording the user data. This process is referred to as xe2x80x9coptimum power controlxe2x80x9d or OPC.
The OPC process is performed before each write operation. However, OPC can produce inaccurate results, because a test write whose length is a fraction of a full revolution of the disk is used to determine the optimum write power. As can be seen in FIG. 1A, the disk 100 includes a small area for test data, labeled Power Calibration Area (PCA) 130 toward the inner portion of the disk 100. The PCA area 130 may be a fraction of a full revolution of the disk 100. This limitation is encoded in the Orange Book Standard for CD-R, which allocates a small area of the media to be used for OPC. The use of power calibration data from less than a full revolution of the disk may cause errors because of any of these issues: vertical and radial run out which cause focus and tracking errors, circumferential variations in the thickness of the recording layer or in the optical properties of the substrate, and localized errors in the test write area.
Additionally, the changes in optimum power for each write speed must be predicted from a minimal sample. As a result of using constant angular velocity (CAV) writing, instead of constant linear velocity (CLV), the spindle speed remains constant, and the tangential velocity of the recording head varies by more than 2:1. Because the test area and number of test writes is limited, as is the speed, the OPC for CAV writing predicts the optimum power for the entire range of write speeds by extrapolation based on one or two short test writes at the ID of the disk. Therefore, OPC has problems in setting the laser power and write format optimally.
FIG. 1B is a graph illustrating the asymmetry v. radius for fixed power write of a compact disk.
The xe2x80x9cactivexe2x80x9d area of the disk, the area where information can be stored, ranges from I.D. (inside diameter)=4.6 cm to O.D. (outside diameter)=11.7 cm. As can be seen from FIG. 1B, the level of asymmetry from the inside diameter (ID) to the outside diameter (OD) varies due to irregularities in the thickness of the recording layer of the media. Traditional OPC, which is done at the ID, cannot predict the optimum power over the entire disc surface.
FIG. 1C is a graph illustrating the variation of the asymmetry as a function of angular position for fixed power write of a compact disk. As can be seen from FIG. 1C, with constant write power, the asymmetry changes cyclically over one revolution of the disk. Hence, the traditional OPC procedure, which records data with fifteen different power levels, each of which occupies only a fraction of a revolution of the disk, is subject to error.
One prior art method of solving the issues of OPC is by using running optimum power control (ROPC). During the recording of user data, the amplitude of light reflected from the media during the writing of the marks that encode the user data may be monitored. The signature of the data, as it is being written, is compared to the data recorded during the OPC process. If the signature has changed, significantly, the power or write strategy is changed.
However, ROPC also has some shortcomings. ROPC adjusts the write power based on the read back data of the reflected write power. The predictive parameters are non-linear and have a low signal-to-noise ratio. They also may vary according to the manufacturer""s process. This reduces the accuracy of the ROPC process.
A method and apparatus for improved laser control during writing. The method comprises setting characteristics of a write laser in a power calibration area of a disk and writing data to the disk using the characteristics set at the power calibration area. The method further comprising halting recording at a stopping point, prior to an end of the disk, and reading the data recorded on the disk. The method further comprising evaluating the data and adjusting the characteristics of the write laser, if appropriate, and restarting the writing using the adjusted characteristics for the write laser.